1. Field of the Invention
The present invention is related to a silicon germanium (SiGe) heterojunction bipolar transistor (HBT) and more particularly to improving SiGe HBT circuit performance.
2. Background Description
A typical bipolar transistor is a vertical stack of layers semiconductor material of alternating conduction type, i.e., NPN or PNP. Normally, the collector is located at the bottom of the stack with the base is sandwiched between the collector and the emitter. Forward biasing the base to emitter junction cases current to flow through that junction. Current through the base to emitter junction cases a much larger current to flow between the collector and emitter. Typically, each transistor terminal (base, emitter and collector) exhibits some inherent internal resistance. Current through an internal collector resistance, base resistance, or emitter resistance reduces the effective voltage to the respective transistor terminal. So, current flowing through any internal transistor resistance develops some voltage that reduces drive and impairs transistor performance. Consequently, internal transistor resistance is a primary limitation on performance for state of the art bipolar circuits.
Silicon Germanium (SiGe) heterojunction bipolar transistors (HBTs) have found widespread use in high speed applications and, especially in Radio Frequency (RF) applications, high speed wired data transmission, test equipment, and wireless applications. Extrinsic Base (ExB) resistance (Rb) has been a primary limitation of transistor performance with HBTs in particular. One way to reduce Rb is to increase the extrinsic base dopant concentration, i.e., implanting a suitable dopant into the extrinsic base to drive the resistance down. For example, the extrinsic base may be implanted with a high concentration boron (B) for a typical NPN HBT. Implanting the extrinsic base also can improve the link between a polycrystalline extrinsic base and a monocrystalline intrinsic base (IB), that is doped with a lower concentration of the same type dopant.
Unfortunately, implanting dopant introduces defects to significantly increase the number of defects in the implanted material with the increase proportional to implant dose, energy, mass and etc. Also, activating the implanted ions (boron) to diffuse them into the extrinsic base, induces interstitials in the extrinsic base to migrate/diffuse into the intrinsic base, which may widen the base. Furthermore, the interstitials also may migrate from the base into active transistor regions, i.e., into the sub-collector, causing sub-collector dopant (e.g., Phosphorous, arsenic, antimony or a combination thereof) to diffuse laterally, increasing internal sub-collector resistance. This interstitial migration reduces HBT gain to further degrade device performance, both static and dynamic (AC/D) performance, dissipating any improvement that might have resulted from reducing Rb.
Thus, there is a need for reducing extrinsic base resistance in high frequency SiGe HBTs without impairing transistor performance.